Several methods are known by means of which a web can be produced by depositing a gaseous suspension of fibers or other particles onto a continuous web-forming belt. For example, U.S. Pat. No. 3,071,822 describes a method in which the fibers are deposited through the intermediary of an oscillating nozzle, which is caused to traverse backwards and forwards across the belt with the aid of mechanical devices. This arrangement is encumbered with a number of drawbacks. The oscillating frequency of the nozzle is restricted to about 1-2 oscillations per second. It is difficult to achieve suitable oscillatory movement that will provide uniform distribution of material over the continuously moving web-forming belt.
U.S. Pat. No. 4,099,296 describes another arrangement which comprises a distribution chamber and a nozzle assembly which discharges into the chamber. The nozzle assembly has an elongated aperture which extends in the longitudinal direction of the forming belt. Arranged on at least one side of the nozzle assembly is a supply means having openings or jets which face the incoming stream of fibers and through which there is delivered pulsed jets of steering gas, the pulses of which are variable. The incoming stream of fibers is subjected to powerful impulses from the steering jets, which disperse the fibers, or material, throughout the distribution chamber in the form of fiber curtains, which are deposited onto the continuously moving belt or like carrier surface. The frequency at which the steering jets change the direction of the fiber stream is higher than in the case of the mechanical arrangement, e.g. from 5 to 15 times per second.
U.S. Pat. No. 4,197,267 is an improvement on the method of the above-mentioned patent and describes a particularly advantageous arrangement for achieving uniform distribution of the fibers, or material, issuing from the nozzle. This is effected by causing the flow of material to pass a zig-zag transition zone located upstream of the nozzle, as seen in the flow direction, and diverging towards the nozzle. The transition zone increases in area in a direction towards the nozzle, therewith resulting in a velocity decrease of the incoming flow of material. Passage of the material flow through the zig-zag transition zone results in uniform distribution of the material in the longitudinal direction of the nozzle.
The arrangements described and illustrated in the aforementioned patents have been found very effective and provide excellent results with respect to the uniformity of the web formed and the general quality of the web. The arrangements, however, do not provide the same good results when producing very thin webs having density below 500 g/m.sup.2, and particularly densities below 400 g/m.sup.2. Webs of uneven thicknesses are obtained at such low densities. In addition, thick regions are formed, presumably due to the fact that fiber bundles are created as the fibers are conveyed to the nozzle, and to the actual distribution chamber. Furthermore, fiber coatings which form on the walls of the distributing chamber are liable to loosen and fall onto the formed web. A milling operation is undertaken in the case of thicker webs.